1. Field of the Invention
The present invention relates to a crystallization technology, and more particularly to an apparatus and a method which form or generate a crystallized semiconductor film by irradiating a polycrystal semiconductor film or an amorphous semiconductor film with a laser beam phase-modulated by using a phase shifter, and to a phase shifter.
2. Description of the Related Art
In general, materials of a thin film transistor (TFT) used for, e.g., a switching element which controls a voltage to be applied to pixels in, e.g., a liquid crystal display (LCD) are roughly classified into amorphous silicon and polysilicon.
The polysilicon has higher electron mobility than that of the amorphous silicon. Therefore, when a transistor is formed by using the polycrystal silicon, a switching speed becomes higher than that when using the amorphous silicon, and a response of a display thereby becomes rapid. Further, a peripheral LSI can be also constituted of such thin film transistors. Furthermore, there is an advantage of reducing a design margin of any other component. Moreover, when incorporating peripheral circuits such as a driver circuit or a DAC other than a display main body in a display, these peripheral circuits can be operated at a higher speed.
The polycrystal silicon consists of an aggregate of crystal grains, and its electron mobility is lower than that of crystallized silicon or monocrystal silicon. In a small transistor formed by using polycrystal silicon, irregularities in crystal grain boundary number in a channel portion are a problem. Thus, there has been recently proposed a crystallization method to form crystallized silicon with a large particle size in order to improve the electron mobility and reduce irregularities in crystal grain boundary number in a channel portion.
As this type of crystallization method, there is known “phase control ELA (Excimer Laser Annealing)” which forms a crystallized semiconductor film by irradiating a line type phase shifter approximated in parallel with a polycrystal semiconductor film or an amorphous semiconductor film with an excimer laser beam. The detail of the phase control ELA is disclosed in, e.g., “Surface Science Vol. 21, No. 5, pp. 278-287, 2000”.
In the phase control ELA, there is generated a light intensity distribution having an inverse peak pattern or patterns that a light intensity is substantially zero or minimum in a line corresponding to a phase shift portion of a phase shifter (pattern that a light intensity is minimum at the center and it is suddenly increased toward the periphery), and a polycrystal semiconductor film or an amorphous semiconductor film is irradiated with a light beam having this light intensity distribution with the inverse peak pattern. As a result, a temperature gradient is generated in a fusing area in accordance with the light intensity distribution, a crystal nucleus or nuclei are formed at a part or parts which solidify first in accordance with a point or points on a line where the light intensity is minimum or substantially minimum, and a crystal grows towards the periphery from the crystal nucleus in a lateral direction (lateral growth), thereby forming a crystal nucleus with a large particle size.
As shown in FIGS. 1A and 2A, the line type phase shifter is constituted of two types of rectangular areas 101a and 101b having different thicknesses which are alternately repeated along one direction (lateral direction) of a transparent substrate 100, and a phase difference of π (180 degrees) is given to a gap between the adjacent areas, i.e., a step portion 101c. 
As shown in FIG. 1B, a light beam which has passed through such a phase shifter forms a light intensity distribution with an inverse peak pattern that a light intensity is minimum on a line 102 corresponding to a phase shift portion (boundary) 101c and the light intensity is one-dimensionally increased toward the periphery in a direction orthogonal to the line 102. FIG. 2B concretely shows this light intensity distribution.
When a processed substrate is irradiated with the light having such a light intensity distribution, a temperature distribution becomes lowest along the line 102 corresponding to the phase shift portion 101C, and a temperature gradient (indicated by each arrow in the drawing) is generated in a direction orthogonal to the line 102 corresponding to the phase shift portion. As a result, each crystal nucleus 103 is generated on the line 102 corresponding to the phase shift portion 101C as shown in FIG. 1C, and crystallization advances, in a lateral direction, that is a direction orthogonal to the line 102 corresponding to the phase shift portion from that crystal nucleus 103. In FIG. 1C, curves 104 indicate a crystal grain boundary, and crystal grains are formed in an area defined by these crystal grain boundaries 104.
In such a crystallization method, although the crystal nucleus 103 is generated on the line 102 corresponding to the phase shift portion 101C, a position on the line 102 where the crystal nucleus 103 or nuclei are generated is uncertain. In other words, when the line type phase shifter is used, any position at which the crystal nucleus 103 is generated cannot be controlled by the phase shift portion, and it is hence impossible to two-dimensionally control a crystal formation area.
Additionally, in the light intensity distribution obtained by using the line type phase shifter, as shown in FIG. 2B, it is general to involve irregular undulations in a middle portion PM between two adjacent inverse peak pattern portions PP (wavelike distribution that an increase and a decrease in light intensity are repeated). In this case, a crystal nucleus 103b may be generated at a position where the light intensity is low in undulations in the middle portion MP (i.e., at an undesired position) in some cases. Further, even if a crystal nucleus is generated at a desired position, the lateral growth which has started from the crystal nucleus toward the periphery is stopped at a part where the light intensity is decreased at the boundary between the inverse peak pattern portion and the middle portion, which prevents growth of a large crystal.